This invention relates generally to fluorescent lamps and is particularly directed to an improved fluorescent lamp start-up circuit.
The operation of most fluorescent lamps in use today which are operated at standard frequencies, i.e., 50-60 Hz, is controlled by an electromagnetic type of ballast. The ballast initiates and sustains lamp operation over a wide range of operating and use conditions. There are primarily three different approaches used for initiating fluorescent lamp operation which are referred to as switch start, instant start, and rapid start. Switch start ballasts typically preheat the lamp electrodes during the starting process, but do not apply any supplementary cathode heating once steady state lamp operation is realized. Instant start ballasts take another approach in that, while they also do not provide supplementary cathode heating during steady state operation, this type of ballast provides no electrode preheating prior to start up. Instant start ballasts depend solely upon the application of a high voltage across the lamp electrodes to provide the necessary starting and operating conditions.
The most common technique and the present trend in both magnetic and electronic ballasts for starting fluorescent lamps makes use of the rapid start approach. In this approach, the lamp and cathode voltages are increased to a fixed, predetermined voltage level which is maintained until the lamp ignites. Rapid start ballasts typically have separate cathode voltage windings integral with their design to allow the lamp electrodes to be heated during start-up and to remain heated during normal, steady state operation. This technique relies upon a balance of both cathode voltage and lamp voltage wherein the lamp voltage is typically raised to a value which will not start the lamp until the cathode is heated to a predetermined temperature. The lamp voltage is therefore limited to a peak voltage which will not cause the lamp to ignite too soon, while the cathode voltage is inversely proportional to the time it takes for the cathode to be heated to a predetermined temperature. The lamp starting time is therefore a function of both the lamp and cathode voltages.
This rapid start approach for initiating the operation of fluorescent lamps suffers from various limitations and is characterized by several undesirable operating characteristics. For example, the lamp voltage required to ignite a fluorescent lamp is a function of the type of fluorescent lamp, its operating temperature and age, and the fixture within which the lamp is incorporated. In addition, similar fluorescent lamps of the same type produced by different manufacturers typically exhibit different lamp voltage ratings and operating characteristics. The interdependence of the lamp and cathode voltages requires a delicate balancing between these two operating parameters in a rapid start ballast. When the ballast is of the electronic type, leakage to the fixture as well as through the lamp tends to upset the balance between these two operating voltages making rapid start operation of the fluorescent lamps even more difficult to achieve.
The present invention avoids the aforementioned difficulties of the prior art by applying the lamp and cathode voltages in a predetermined, timed manner for reliably initiating operation of fluorescent lamps of various types and manufacturing brands having a wide range of operating temperature and wattage requirements.